GCHandle.FromIntPtr(IntPtr) 方法

定義

命名空間:

System.Runtime.InteropServices

組件:

System.Runtime.dll

Source:

GCHandle.cs

傳回新的 GCHandle 物件，而此物件是從 Managed 物件的控制代碼所建立。

public:
 static System::Runtime::InteropServices::GCHandle FromIntPtr(IntPtr value);

public static System.Runtime.InteropServices.GCHandle FromIntPtr (IntPtr value);

static member FromIntPtr : nativeint -> System.Runtime.InteropServices.GCHandle

Public Shared Function FromIntPtr (value As IntPtr) As GCHandle

參數

value

IntPtr

nativeint

Managed 物件的 IntPtr 控制代碼，要從此控制代碼建立 GCHandle 物件。

傳回

GCHandle

新的 GCHandle 物件，其對應於實值參數。

例外狀況

InvalidOperationException

value 參數的值為 Zero。

範例

下列程式碼範例示範使用 App 方法建立 Managed 物件的 GCHandle.Alloc 控制碼的類別，以防止收集 Managed 物件。 對 方法的 EnumWindows 呼叫會傳遞委派和 Managed 物件， (宣告為 Managed 型別，但未顯示) ，並將控制碼 IntPtr 轉換成物件。 Unmanaged 函式會將類型傳回給呼叫端，做為回呼函式的參數。

using System;
using System.IO;
using System.Threading;
using System.Windows.Forms;
using System.Runtime.InteropServices;

public delegate bool CallBack(int handle, IntPtr param);

internal static class NativeMethods
{
    // passing managed object as LPARAM
    // BOOL EnumWindows(WNDENUMPROC lpEnumFunc, LPARAM lParam);

    [DllImport("user32.dll")]
    internal static extern bool EnumWindows(CallBack cb, IntPtr param);
}

public class App
{
    public static void Main()
    {
        Run();
    }

    public static void Run()
    {
        TextWriter tw = Console.Out;
        GCHandle gch = GCHandle.Alloc(tw);

        CallBack cewp = new CallBack(CaptureEnumWindowsProc);

        // platform invoke will prevent delegate to be garbage collected
        // before call ends

        NativeMethods.EnumWindows(cewp, GCHandle.ToIntPtr(gch));
        gch.Free();
    }

    private static bool CaptureEnumWindowsProc(int handle, IntPtr param)
    {
        GCHandle gch = GCHandle.FromIntPtr(param);
        TextWriter tw = (TextWriter)gch.Target;
        tw.WriteLine(handle);
        return true;
    }
}

Imports System.IO
Imports System.Threading
Imports System.Windows.Forms
Imports System.Runtime.InteropServices
Imports System.Security.Permissions

Public Delegate Function CallBack(ByVal handle As Integer, ByVal param As IntPtr) As Boolean


Friend Module NativeMethods

    ' passing managed object as LPARAM
    ' BOOL EnumWindows(WNDENUMPROC lpEnumFunc, LPARAM lParam);
    <DllImport("user32.dll")>
    Friend Function EnumWindows(ByVal cb As CallBack, ByVal param As IntPtr) As Boolean
    End Function
End Module


Module App

    Sub Main()

        Run()

    End Sub

    <SecurityPermission(SecurityAction.Demand, UnmanagedCode:=True)>
    Sub Run()

        Dim tw As TextWriter = Console.Out
        Dim gch As GCHandle = GCHandle.Alloc(tw)

        Dim cewp As CallBack
        cewp = AddressOf CaptureEnumWindowsProc

        ' platform invoke will prevent delegate to be garbage collected
        ' before call ends
        NativeMethods.EnumWindows(cewp, GCHandle.ToIntPtr(gch))
        gch.Free()

    End Sub


    Function CaptureEnumWindowsProc(ByVal handle As Integer, ByVal param As IntPtr) As Boolean
        Dim gch As GCHandle = GCHandle.FromIntPtr(param)
        Dim tw As TextWriter = CType(gch.Target, TextWriter)
        tw.WriteLine(handle)
        Return True

    End Function
End Module